


Brush up on these Bones

by Rehlia



Category: Undertale (Video Game)
Genre: Alternate Universe - Underswap, Cleaning Kink, Guilty Pleasures, Insecurity, Kink Exploration, Non-Sexual Kink, Oneshot, Other, Sensitive bones, Shame, Solo, Underswap Sans, brushing kink, weird kinks
Language: English
Status: Completed
Published: 2018-05-19
Updated: 2018-05-19
Packaged: 2019-05-09 01:04:36
Rating: Mature
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,917
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/14706206
Author URL: https://archiveofourown.org/users/Rehlia/pseuds/Rehlia
Summary: When his brother isn't home, Blue likes to play with his toys.They maybe aren't quite the ones you'd expect.





	Brush up on these Bones

“Pap?”

His voice echoed through the house in a certain way that he knew to mean it was empty, but he had to make _sure_. 

“Papy?”

He made sure to check everywhere systematically. He put his boots away so he wouldn't trail flaking dirt all over the house, or at least not quite as much of it, seeing that his feet were also dirty, moving from room to room. The kitchen. The living room. The upper hallway. The bathroom. Both of the bedrooms (after knocking at his brother's door, of course!)

But no; his brother seemed to have stuck to his plan of visiting Undyne this afternoon. 

Sans breathed out a sigh mostly in spite of himself. He really wanted to be alone right now, although a part of him also felt it would've been better if he weren't. 

Slowly, he moved into his room and opened the cupboard, pulling out a locked box he kept there under a stack of old boots. The key to it was hidden in a hollow pocket in his alarm clock - perhaps an unnecessary security measure, both brothers would always respect a locked box for the plea of privacy it represented and keep away, but Sans had to make _sure_. 

He brought the box to the bathroom, fetched some towels for himself, and then locked himself in. 

Another big sigh escaped him.

He was completely alone. Locked in. Only him and his… tools. 

He looked at himself in the mirror, taking stock of his current state. 

The short version was that he was absolutely caked in mud, filthy from the top of his skull to the tips of his metatarsals. The long version was that this had been one of the most intense training sessions that Alphys had ever put him through, and his appearance had suffered accordingly. Reddish-brown stains covered his skull, marking the places where he had fallen into the dirt. His battle body was partially wet, thanks to one part of Alphys’ obstacle course that included a moat, and the earth of Hotland had mixed with the warm, brackish water into a dark mud that had soaked so deeply into the fabric that he didn't know if he'd ever be able to wash it out again. Worse, the mixture had found its way into the gaps between his bones and partially dried there, irritating the sensitive spots with a grinding sort of itch that was driving him nuts. He knew he trailed dirt through the house even after taking his boots off. He'd have to take care of that… after he took care of himself. 

He pulled his battle body off and dropped the clothing items into the laundry basket, carefully wadded up so they wouldn't stain the other clothes in there too much. Doing that revealed the state of his bones underneath; even there he was covered in filth. The mud had soaked through his clothes thoroughly. Then he stepped into the tub. Thankfully, the bathroom wasn't that big, so he could still see himself in the mirror and easily reach his tools.

Sans felt his hands tremble a bit and shot a guilty glance over at his box. 

Should he really?

But who knew when the opportunity would arise again. 

Swallowing his nerves, he opened the box and took stock of the assortment of brushes and sponges within. It wasn't _unusual_ to have different sponges and brushes to clean oneself with, especially not as a skeleton monster with a lot of gaps and nooks and crannies between sturdy bones to take care of. Even then though, Sans’ collection was something to behold. He probably could have opened his own shop, if he wanted to. 

His soul trembled at the sight, causing him to hesitate once more.

Why did he have to be so weird?

In spite of his feelings, Sans reached out and picked up an oval brush slightly larger than his hand, made of wood with a soft strip of woven cloth to stabilise his grip and stiff, dark bristles. This seemed like a good start, he decided. 

He slid his hand under the cloth strip and brought the brush to his right arm, brushing over the radius and ulna in long, firm downwards strokes. The sound of the brush seemed loud in the quiet bathroom. The dirt that had caked his bones there in multiple partially dried layers began to flake away under the stiff bristles, falling into the tub. It tickled where the brush cleared some of the filth away, and the stiffness of the bristles provided him with a nice scratch on the spots where the drying dirt had itched him. Stars, this was such a relief. 

Moving upwards, he brushed over his humerus, his clavicle, the front of his ribcage, down his spine and then, skipping his pelvis entirely, down both of his legs until he had dragged the brush over the left and right metatarsals. Then he switched over to his nondominant hand to clean his other arm. By the time he was done, he was standing in a semicircle of dried dirt flakes. 

“Gross,” he mumbled, his voice more breathy than he'd like. 

He fished a long piece of wood out of his toolbox, a handle that could be clipped onto the oval brush he was holding. After clipping them together, he brought the brush over his shoulders to start cleaning his scapula. 

A low sigh forced its way out of his mouth before he managed to clench his teeth together again. 

That spot always got him. Right under his scapula, where they covered the ribs. It was hard to reach there, but thanks to the extension on his brush, the stiff bristles could scrub there just right. 

“Oooh…” he breathed out when they caught on the underside of the bone, prying away a stiff bit of dried dirt that had been bothering him immensely ever since he climbed out of Alphys’ moat. 

Quickly, he went on, scrubbed over the ribs on his back and his spine from top to button. When he was done, most of the dried dirt was removed, the crusted parts that stuck to him gone. What was left now were the wet parts that hadn't had the chance to dry yet, and the discoloured stains the dried dirt left on his bones. And of course his pelvis. That was too sensitive for the stiff bristles of this brush though. 

Satisfied with his progress so far, he turned on the water and directed the spray at the bottom of the tub, to get the flakes of dirt softened up until they dissolved and ran down the drain together with the water. Then he cleaned his brush, making sure that the bristles were free of dirt, before he put it on the nearby towel rack to dry. 

Only then did he pick up his next tool from the box and stepped under the spray of water himself. 

He eyed the sponge brush he had picked, the way it had been cut to resemble three thick strands woven together. With the edges this shape created, it was easy to mold the sponge against his bones and still remove the dirt efficiently without having to rely on cleaning substances with harsh chemicals. Instead, he could use his favourite neutral soap, a mild natural product made by a local monster from herbs growing in the Snowdin area. He knew a lot of fleshier monsters thought he used bleach to get his bones so clean and shiny, but that was silly of course. Regular bleach was way too aggressive to use on bone matter, it could damage them. Even hydrogen peroxide, often used to bleach bones of dead animals fallen Underground to display them in laboratories and for school, was too harsh on living bones like his own. It would be painful to use at the very least, might even sap his one HP and dust him in the worst case. But of course monsters who weren't skeletons would have no reason to know that; Sans didn't blame them or the misconception. 

Squeezing a good bit of soap onto his sponge brush, he began to scrub at the stains and wet dirt on his bones. The dirt stained the sponge and the foam began to discolour and became an ugly brown quickly, but at least his bones were left clean in the aftermath. 

The stark difference between dark ochre dirt and pale white bones, a sharp line between one and the other.

Beautiful.

Taking a deep breath, he added more soap to the sponge and began gently running his sponge over his pelvis, clenching his teeth together. 

The warm water of the shower had softened up the dirt so it was easy to get rid of, but after having to endure for so long with the dirt stuck there while he listened to Alphys tell him about all the mistakes he had made in her obstacle course and on the way home… well, it was amazing to get to scrub it away, that was all. 

“Ah…”

The sound became a hiccup halfway through as he struggled to keep himself controlled, and failed. 

The sponge was so soft on his pelvis, the soapy foam cleaning the dirt away so pleasing, and it smelled so nice. Watching the dark brown clinging to him make way for the soft white underneath was _so_ visually satisfying. 

He struggled to control his breath as he approached his public bone, ending up dragging the sponge over the sensitive spot in a hasty manner. Then he did the inside just as fast, not lingering anywhere in particular. 

This was the worst part, always too sensitive. He… he shouldn't even have brought out the box. 

Head hanging, he cleaned the sponge until the water ran clean from it and wished he could be done already, while another part of him wished it would never end. Placing the sponge on the towel rack as well to dry, he reached into the toolbox again. 

This time, he chose a bottle brush, medium soft bristles arranged in a cylindrical way on a long handle. His breath hitched just from looking at it, just from knowing what was to come. 

When the brush made first contact with his ribs his voice skidded past its usual deep register straight into something smoother and higher that almost sounded boyish as he groaned. He hated it, and he also didn't have enough mental capacities left to hate it, because having the bottle brush clean out the gaps between his ribs, _behind_ his ribs, felt just too good. 

The bristles caught on the insides of his sensitive bones, tickling and scratching in the most pleasant way, leaving him clean, clean, clean. It was the greatest feeling, the most luxurious sensation imaginable. A thorough scrubbing like this was one he got to indulge in so rarely… 

He usually didn't have the time, or the privacy. 

And it did use up a lot of hot water. 

But stars, the sensations. 

Sans fumbled as he switched out the brush again, selecting a smaller nail brush now to clean the smaller gaps in his bones. This one could be used in a lot of spaces, like the gaps between his finger bones where he utilised the harder side of the brush and doubled down on the scrubbing. His hands had gotten a lot of the dirt and the grinding of the mud between his joints was particularly annoying here. His hands could take the harshness, and he enjoyed pressing down hard, moving the small brush over his bones with force. He could see the bristles dip into the tiny gaps where his bones were held together by magic, could see them scrape out the small particles of dirt bit by bit, could feel the pleasant pressure and the individual hairs of the bristles as they moved over his bones. 

A whole body shudder overcame him, strong enough that he almost dropped the brush. 

Once his hands were done, he switched over to his feet, giving them the same treatment. Like with his hands, he enjoyed using more force here, to brush with enough strength that it became a massage, and here he didn't feel self-conscious at all when he moaned in pleasure. 

Enjoying a good foot massage was perfectly normal and respectable after all. 

It felt so good to let his pleased sounds out like that that he felt almost reluctant to move on to the next step. 

Another bottle brush was procured from the box, smaller and with much softer bristles. 

His hands shook as he lowered it into his pelvis. 

The sensation of the brush against the inside of his pelvic inlet made his legs shudder, his pelvis rocking forwards towards the source of stimulation without his input as he keened. He began moving the brush back and forth, slowing down at the parts where different bones came together. 

He felt the soft bristles going into his ischia, molding themselves against the gentle curve of the bone. So many fine hairs dragged over the surface, and he could feel them all. He moved the brush over to the inside of his pubis and wailed, his voice rising and sinking oddly with each movement. It was too much and yet not enough, one of his most sensitive body parts stimulated by the tickle of so many thin, soft bristles…

He yanked his hand back, panting heavily and blushing deeply as he stared at his pubis. The scrubbing, soft as it was, had left the sensitive bone somewhat irritated, blotchy patches of blue magic staining the parts where the brush had been. 

Shame rolled over him in waves as he took in the sight. 

Sure, it was his pubis, his pelvis, and that was sensitive, but… this shouldn't be about gratification. It wasn't!

...Was it?

He didn't know and honestly, that was probably part of the problem. This, this cleaning thing, it straddled the line between tickling him with a deep sense of mental but ultimately harmless satisfaction and actual sexual gratification, and he wasn't always sure which side of the line his reaction would fall on. And it wasn't that he had a problem with sexual things, not at all, but… this was should be nonsexual. To think differently was so out of the ordinary. If he got off to the thought of cleaning things, then what did that say about him? Did that make it inappropriate when he cleaned the house? That was such an uncomfortable thought. He had never experienced these feelings when someone else was there, thankfully, it wasn't as if he would start moaning while cleaning his boots with Alphys for example, or that he'd feel heated while scrubbing a pot while his brother dried the dishes. Which Sans was very, _very_ grateful for, because if that happened he might actually dust out of sheer mortification.

But as soon as he was doing it alone?

It became like this, and it made him question everything he thought he knew about himself, and he kind of hated it for that, in spite of how much he loved it. 

The pit of guilt and shame in his soul made him put away his brush and just stand in the shower for a bit. 

His bones may look clean, but he felt filthy inside. Something so harmless, and he made it into something so strange. Why couldn't he just enjoy this in a harmless way? Like a normal monster? Surely Alphys didn't get all squirmy when she took a sand bath. Surely the queen didn't moan when she polished her armour. Surely Grillby didn't feel aroused when he mopped his bar. 

No point in lingering on that. 

He should just finish fast, get the rest of the dirt off and then go clean the mess he left when he walked through the house. 

He still grabbed his facial brush though. 

It wouldn't do to be _sloppy_ just because he wanted to finish this quickly. 

Oh, who was he kidding. In spite of the way guilt and shame burned through his soul, he didn't want to stop. It was too good and he got the chance only so rarely, he didn't want to stop this, whatever it was. 

The facial brush in his hand was an electric one, built and marketed by Napstablook. The robot had originally used it to polish the sensitive metal on his face and then decided to sell it for monsters with similar needs. Sans had immediately bought one of course. It looked a bit like a sleek screwdriver, only with a round brush head at the tip. When he pushed a button, the brush head began to rotate too fast for him to see, sending a faint vibrating sensation through his hand. 

He applied some soap to his face and pressed the rotating brush against it, careful not to get any soap into his eye sockets. 

A pleased grumble resonated through the bathroom, his sockets fluttering shut as he positively melted into the sensation. The mechanical turns of the brush were so fast that it felt more as if it was fluttering against his face, and the bristles of this brush were so soft that there was no scratching sensation at all. Instead it was like the gentlest of massages, a flurry of movement and vibration against the sensitive bone on his skull. He felt much of his tension easing as he moved the brush over the pressure points at his temples and his forehead. Then he moved back over his skull until the brush was massaging the nook where his spine met his cranium. 

“Mmmh…”

It was so relaxing. He wasn't prone to headaches but even so, the even pressure on his skull felt good. It was as though he was letting out a breath he hadn't known to be holding. As if he had been trying to move in clothing that was too tight and restrictive, and now it was removed and he could decompress. He was at ease, with himself and with the world, in spite of his own worries over this act and what it meant to enjoy it so much. 

He ran a hand over his skull when he thought he was done and it felt smooth, the bones of his hand gliding over it so easily.

He put the facial brush aside and took a second to decide between his toothbrushes. A big head or a small one? Hard or soft bristles? And should they be even or of different lengths, feathered at the tip or not? Definitely feathered, that made it easier to get into the gaps. Based on that, a small head. 

Adding toothpaste to the brush, he brought it to his teeth and moved it in small, circular motions. Another groan escaped him, viscerally pleased. His teeth didn't have much feeling to them, but the bone they grew out of and the flexible ridge surrounding them that he could emote with were sensitive. Whenever the toothbrush cleaned the tight gaps between his teeth and the bristles came into contact with those sensitive points, it left him shuddering. These tiny, fine bristles cleaning the dirt out of the tight gaps, and the tickle of sensation that accompanied that… amazing. His favourite spot were his molars, difficult to reach and therefore all the more pleasing when the small head of his toothbrush cleaned out the back of them.

Perhaps he brushed a bit longer than strictly necessary, but it was just so nice. At the end of it, he was left with a soft tingle in his mouth, the taste of fresh mint lingering. He conjured a tongue just to run it over his teeth, relishing how smooth they felt. 

Sans decided he was finally done with his shower and stopped the flow of the water. The bathroom had steamed up noticeably while he was in there, and he couldn't see himself in the mirror anymore. Everything seemed soft and hazy, the air heated and heavy. 

He felt warm and incredibly relaxed. 

Instead of a towel, he picked another tool out of his box, this time a bottle brush topped with noodle-shaped microfiber extensions. He ran it over his body in a similar manner to the brush he had used first to brush the flaking dirt off his body, starting with his hand and then moving upwards over the arm, his ribcage, spine, legs, feet… only this time he included his pelvis, and all the gaps between his bones. 

The microfiber dried his bones effectively and more than that, it polished the surface to a shine. It was so terribly soft on him, softer than almost anything else that ever came into contact with his body. 

Sans closed his sockets and allowed himself to become lost in the sensations. He imagined that the softness of the microfiber dipping in and out of the gaps between his ribs was another monster caressing him, or that he was underwater and brushing against plants in a current as if he was a fish, or that he was drifting in a cloud. Harmless fantasies that never went anywhere other than the first concept, and yet they left him deeply fulfilled. It was so peaceful that even when he moved the microfiber into his pelvis, he moaned only a little, barely feeling the inevitable arousal to the stimulation. 

He was done. 

His eyes opened. 

The moisture in the air had evaporated a little, enough that he could see his form in the mirror again, although still blurry and undefined. Even so his bones were gleaming, looking almost pearlescent under the light of the bathroom. 

Sans felt good. Relaxed and calm and fulfilled. 

_Clean._

His eye lights looked soft and fuzzy in the mirror, and not just because the last of the condensation hadn't evaporated yet. 

He twisted the microfiber brush in his hands. Was it really so bad if he allowed himself to indulge in this if it made him feel so good? He still didn't know. He was worried that at some point it would spill over, bring these feelings he was having from the realm of privacy into his interactions with others. That he would some day sit down to brush his boots with Alphys and feel the tingle in his soul. That he would have to avoid his brother while cleaning things. He was scared of that. But then again, denying himself felt terrible too. It made him antsy and sad. 

With a sigh, he put the brush with the others onto the towel stand. A blow dryer ensured that they were robbed of the last remnants of moisture that might have clung to them before he carefully packed them away, locking them in their box once more. Then he opened the window before he left the bathroom, to air out the room. His box went back into his wardrobe, the key back into that hidden pocket of his alarm clock.

The house was still silent, which didn't surprise him. He had certainly taken his time in the bathroom, but his brother tended to take his time when he hug out with Undyne. It was entirely possible that he'd stay there until it was late at night, or that he'd even stay over. 

It was nice because it awarded Sans more time to bring out his shoe brushes and the steamer, to clean the floor and his boots… 

Privately.


End file.
